After years of halfheartedly picking at this problem, I finally discovered the solution. I always knew REX - Regina was a tempermental beast, but I really had no idea...

The problem was with my wife's 940. One day I noticed it had a severe miss at high RPM (~5K or above.) It was bad enough that the engine wouldn't advance beyond that RPM. It did not seem to be load-dependent, as it would do it on the road, as well as in the garage in Park. This wasn't an urgent problem, since my wife doesn't usually use that part of the powerband, but it bugged me. After replacing plugs, cap, rotor, wires, coil pack, RPM sensor, FPR, adding grounds, cleaning connectors, tracing signals, swapping parts with a buddy's Regina car, etc...etc...etc... for the last two years, I became determined to find the cause once and for all. By now, it was almost certainly down to a bad ECU I thought. Then I saw this little tidbit over at Turbobricks:

"Rex notes:
It relies *heavily* on its o2 sensor (which is different from most other narrow bands that i've seen, but this could merely be a coincidence)
the slightest bend in the trigger wheel on the flex plate will yield wild results with the ecu, including but not limited to : it firing only on two cylinders (whoops) and/or it throwing a check engine code and reverting to fail safe ignition timing (bastard)."

The light went on. Some time ago, when I was replacing the timing belt on the 940, I didn't look where I was wedging my big screwdriver and I slightly bent the tone ring. Slightly, as in barely noticeable, as in pushed in a couple of mils further than normal. I thought nothing of it at the time, but last night I went in and gently bent it back out. Bingo - no more miss.

Two years of hair pulling over a screwdriver slip... I LOVE YOU REX!